1. Field of the Invention
The present invention relates to a rubber composition including biodegradable aliphatic polyester and to a pneumatic tire using the composition.
2. Description of the Background Art
In recent years, from the viewpoint of environmental problems such as tightening regulations for resource conservation and prevention of carbon dioxide (CO2) exhaust, importance is attached to the fuel efficiency of tires and coexistence with the fuel efficiency and tire characteristics, such as grip performance, is an urgent task. For improving the fuel efficiency, many methods using silica and a silane coupling agent have been reported. However, performance requirements have not been satisfied.
Also in the used tire recycling field, material recycling in which rubber chips or a rubber powder of tires are reused directly as rubber is demanded to be practically applied. As a method of such recycling, a method including application of a strong shear force to rubber chips or a rubber powder by a biaxial extruder or the like, followed by pulverization and desulfurization is studied, for example. Such physical treatment, however, has a problem of needing a large amount of energy. Under such circumstances, as methods of material recycling using less energy, there are many proposals of methods for decomposition of various types of rubber using microorganisms. Such methods using microorganisms, however, have a poor practical usability problem due to, for example, a long treatment time. Japanese Patent Laying-Open No. 2004-099738 proposes a method of degrading vulcanized rubber compositions contained in waste rubber products like waste tires by use of wood-rotting fungus. By this method, waste tires can be decomposed and disposed relatively easily, but there is no consideration for improvement in properties of tires in Japanese Patent Laying-Open No. 2004-099738.
Similarly in the resin industry, development of resin which can be decomposed with time by the action of microorganisms or the like, i.e., biodegradable resin, is in progress from the viewpoint of environmental protection. Like conventional plastic materials, such biodegradable resins can be used for the production of various products by melt processing or the like. In addition, biodegradable resins have an advantage that they are degraded by microorganisms in nature. They, therefore, do not remain in the natural environment and do not cause environmental pollution unlike many conventional organic polymer compounds. However, there is a problem that since resin does not have flexibility like that of rubber, it is difficult to use resin for rubber compositions for tires.
Further, regarding tires containing natural resources, a method using a starch composite has been proposed, for example. This method, however, needs a mixing step at high temperature for fully dispersing the starch composite and, therefore, the process is not only complicated but the degradation of polymers is accelerated.